In this work, we extend a spectral inversion method for determining the properties of a thin bed and those of the media lying above and below it from prestack seismic data. These properties include the thickness of the thin layer as well as the compressional- and shear-wave velocities and densities of the three media. The estimation of the model parameters is carried out in the frequency domain, and can be applied to the characterization of geological formations with thicknesses below tuning. We apply the methodology to noisy synthetic data generated by considering different thin layers representative of gas reservoirs and having thicknesses of only a few meters. The corresponding results indicate that, under certain conditions, reasonable solutions can be obtained when using conventional prestack seismic data (PP waves). Moreover, we show that the additional information provided by the converted shear waves (PS waves) allows us to improve the quality of the results and, at the same time, it enables us to relax the constraints required when using PP waves only. Finally, in order to reduce the nonuniqueness difficulties typically arising in this kind of inverse problem, the solutions are forced to honor, within a certain tolerance, the potential correlations between P- and S-wave velocities, as well as between P-wave velocity and density, which constitutes valuable information often available from well log data. As a consequence, the uncertainties associated with the estimates of the thickness, wave velocities and density of the thin layer are significantly reduced, therefore allowing to obtain more accurate and reliable solutions.